In a radio frequency identification (RFID) system, RFID transponders or tags are usually mounted onto objects to provide information upon receiving interrogating signals transmitted by a transceiver. In the art, such a transceiver is referred to as a reader. Information provided by an RFID tag upon activation by an interrogating signal can include status, location, or presence of an object to which the RFID tag is mounted. Typically, such information is provided either with data extracted from a semiconductor device of the RFID tag or when the RFID tag interacts with an RF signal to generate a response signal back to the reader.
An RFID system has many different applications. For example, one common application of an RFID system is in electronic article surveillance (EAS) to detect movement of items within surveilled area. Typically, in such an RFID system, an active RFID tag attached to an item is deactivated when movement is authorized. Otherwise, if the active RFID tag is not deactivated, then an alert signal is generated by the RFID system when the item with an active RF tag is conveyed into an area where interrogating signals are present.
Interrogating signals transmitted to an RFID tag are received via an antenna. Such an antenna typically forms part of a resonant circuit of the RFID tag. Generally, the resonant circuit is tuned to a resonant frequency corresponding to the frequency of the interrogating signals. FIG. 1 illustrates a prior art RFID tag 10 having a substrate 12 with a semiconductor device 14 that is coupled to coil windings 16.
A problem with existing RFID tags is that objects with electromagnetic properties adversely affect resonant frequencies of these existing RFID tags. One technique of overcoming this problem is described in U.S. Pat. No. 5,276,431 Piccoli et al. in which a security tag has a resonant circuit that compensates for inherent capacitance of an article to thereby shift a resonant frequency of the resonant circuit closer to the center frequency of a detection frequency range. However, the resonant circuit described in U.S. Pat. No. 5,276,431 Piccoli et al. provides for a predetermined detection frequency range and, therefore, is not optimized for accurate detection of a specific detection frequency.
Another prior art RFID tag is described by U.S. Pat. No. 5,541,399 de Vail in which an RF transponder is fabricated on a single side of a substrate and has a lead line that yields capacitances in crossing over windings of an antenna for the RF transponder. Although this technique reduces cost of the RFID transponder that is easier to attach to many different types of packages, the total capacitance provided at crossover sites along the lead line is limited because of the width of the antenna windings. Hence, a discrete capacitor may still be needed to increase the total capacitance to attain a desired antenna resonant frequency.
Although the above existing RFID tags overcome various problems, there is still a need for a more effective RFID tag that is easier to manufacture and has a wider capacitive range without having to use discrete capacitors. Also, metallic portions of some objects or products are known to interfere RF or electromagnetic fields and this reduces effective range of RFID tags mounted to such objects. For example, metallic portions in compact discs and the like are known to disrupt RF or electromagnetic fields around RFID tags attached to the compact discs.
Furthermore, RF or electromagnetic fields of an RFID tag are typically weakened when the RFID tag is mounted to a metallic product such a compact discs and the like and housed within a carrier. In this situation, the RFID tag is less effective and this adversely affects sensing range of a reader that interrogates the RFID tag.
Therefore, what is needed is RFID tag that overcomes poor RF or electromagnetic fields when mounted to an object having metallic portions. What is also needed is a tag coupler for an RFID tag mounted to a metallic product to thereby improve detecting range of a reader that interrogates the RFID tag when the product is housed within a product carrier.